Field of the Invention
The present invention relates to a method of removing homogeneous catalysts from organic reaction mixtures.
Discussion of the Background
Methods of recovering catalysts from reaction mixtures are of fundamental importance for the economics of industrial chemical processes since without these methods it would be impossible to reuse often costly catalyst material. Yet, the recovery of catalysts presents those skilled in the art with particular challenges, particularly when homogeneous catalysis is concerned. Since in homogeneous catalysis the catalyst and the reactants are in the same phase it is usually impossible to separate the catalyst from the reactants by simple physical separation methods, for example centrifugation or conventional filtration.
One way to recover homogeneous catalysts is using nanofiltration. Nanofiltration is a pressure-driven membrane method wherein a specific membrane is used to remove selected dissolved components from a liquid phase. The selectivity of the membrane may be based on various mechanisms. In the size exclusion mechanism, the retained dissolved components are prevented from permeating the membrane by steric effects. Removal thus depends on the size of the dissolved component and the mean pore size of the membrane. Here, the membrane is usually characterized by the molecular weight of the retained components. In an electrostatic mechanism, the selectivity of the membrane arises from the surface charge on the membrane and the charge on the dissolved components. A charge of the same sign causes electrostatic repulsion and thus retention of the dissolved components. It is thus also possible, for example, to employ nanofiltration to remove heavy metal ions from aqueous solutions. Finally, separation may also be based on the membrane forming a separate phase which dissolves the constituents of the mixture to be separated. Separation is then due to the different solubilities and diffusion rates of the components. The resulting transport rates of individual components may vary to such an extent that said components are depleted in one another, i.e., separated. The separation effects utilized in a membrane are thus distinctly more complex than the purely mechanical screening effect utilized in filters.
Membrane separation is employed in water and wastewater treatment for example. One problem with using nanofiltration in other environments is often the poor stability of the membrane in solutions other than aqueous solutions. The stability of the membrane towards organic solvents in particular is often insufficient. The membrane moreover utilizes a wide variety of interactions with the medium to be separated to perform its separation duty. Choosing a membrane material suitable for a particular separation duty is therefore anything but trivial.
Chowdhury et al. employ a ceramic membrane to recover homogeneous catalysts from organic solvents in the epoxidation of olefins:                S. R. Chowdhury et al., “Recovery of homogeneous polyoxometallate catalysts from aqueous and organic media by a mesoporous ceramic membrane without loss of catalytic activity”, Chemistry—A European Journal, 2006, Vol. 12, Issue 11, pp. 3061-3066        
A membrane particularly suitable for use in the presence of epoxidized cyclic unsaturated C12 compounds with hydrogen peroxide has not hitherto been identified.